1. Technical Field of the Invention
This invention is concerned with the use of an array of generically different seismic acoustic sources to enhance the initial acoustic impulse at the expense of caudal transients.
2. Discussion of the Prior Art
The use of multiple seismic acoustic sources in seismic exploration on land and at sea is well known. Usually, a number of individual sources are grouped in an array of prescribed dimensions. One purpose of an array is to cancel undesired horizontally-travelling surface waves by destructive interference and to additively reinforce the downwardly-travelling acoustic wavefront from which reflected signals are derived. Typically, all of the seismic sources within a given array belong to the same genus although different species of sources, chosen from the same genus, may be employed.
For purposes of this disclosure, seismic acoustic sources that function according to the same operating principle are considered to fall within the same genus. Sources characterized by different sizes or designs, but having a common operating principle, are considered to be species within the genus. Thus, air guns belong to one genus, sparkers or arcers belong to another genus, imploders to a third, explosive-gas guns to a fourth genus and so on.
As is well known to geophysicists, the pressure signature, that is, a recording of pressure vs time of an outgoing acoustic signal, consists of a strong first impulse followed by a caudal pulse train or tail of weaker pulses. The pressure signatures attributable to sources selected from within any one genus, have similar patterns that are characteristic of that genus. The amplitudes and spacings-in-time of the caudal pulses generated by two species of sources from the same genus may vary a little bit with respect to each other. But the signal pressure-signature of the one species can be scaled such that it can be correlated, to a high degree of certainty, with the signal pressure-signature of another species of the same genus.
On the other hand, pressure signatures of acoustic signals emitted by seismic sources selected from different genera are completely incoherent with one another except for the first impulse. That is, the pressure signatures are not correlatable within any significant degree of certainty. Illustrations of the pressure signatures of signals emitted by various genera of seismic acoustic sources are shown in a handbook entitled "Seismic Energy Sources 1968 Handbook", edited by F. S. Kramer, R. A. Peterson and W. C. Walter, published by the United Geophysical Corporation, Division of Bendix, Pasadena, Calif.
The presence of a caudal pulse train following the initial impulse as generated by a seismic sound source, is very undesirable because each of the caudal pulses acts as a new impulse. The resulting seismic reflection recording becomes complex and difficult to interpret. Accordingly it is important that the initial impulse be enhanced and that the unwanted caudal pulses be suppressed.
As discussed above, use of a number of sources of the same genus, such as air guns, deployed in an array and fired simultaneoulsy or in a selected sequence, will additively reinforce the desired initial impulse. The various sources may be of different sizes so that the power or output signal strength of the respective sources varies according to some selected weighting function. As a result of weighting, the characteristics of the caudal pulse trains emitted by the varously-sized sources may vary a little bit with respect to each other as earlier discussed. But in the composite seismic signal generated by the array, the undesired caudal pulses will not, in general, cancel (complete cancellation being the ultimate desideratum). At best, they will only have a tendency not to reinforce each other relative to the desired initial pulse.
The literature abounds with descriptions of source arrays and configurations designed to suppress the caudal pulse train relative to the initial impulse. Two patents of interest are U.S. Pat. No. 3,602,878 to Sullivan for "A Method and Apparatus for Generating Seismic Waves" and U.S. Pat. No. 3,893,539, to Mott-Smith for a "Multiple Air Gun Array of Varied Sizes with Individual Secondary Oscillation Suppression." Use of a three-gun array is also discussed in the United Geophysical Corporation Handbook cited supra.
In another aspect of the prior art, it is well known that shallow earth layers are most readily detected by use of seismic acoustic sources rich in relatively high frequencies (100 Hz and up). On the other hand, deeper strata are best surveyed using relatively low frequency sources in the range of about 10-40 Hz. In the course of a single survey, both shallow and deep earth layers may be surveyed by using two sources, each having a different dominant frequency content. The two sources are fired separately, first one and then the other in a desired alternate sequence. The seismic reflection returns resulting from the alternate firings of the separate sources are recorded in corresponding separate records. The separate records are not combined. The two (or more) sources are usually different species of the same genus such as a large and a small air gun. For example, see U.S. Pat. No. 3,744,021 to Todd for an "Offshore Seismic Exploration Method".
The two sources may also be selected from different genera. See for example U.S. Pat. No. 3,351,899 to Luehrman for a "Programmed Multiple Shot Source System and Method." Luehrman provides an exploding-gas gun and an arcer or sparker. The two sources are separately fired at programmed intervals, in alternate sequence. Separate seismic recordings are made corresponding to each source. Considerable pains are taken in the above patent to record reflection data from one source or from the other source, but not from both at the same time. In fact, Luehrman teaches that it is highly detrimental to fire the sources substantially simultaneously.